1. Field of the Invention
The present invention relates to a thin-film transistor (TFT) used in an active matrix type display or the like, and to a method for manufacturing a thin-film transistor.
2. Description of Related Arts
In recent years, polycrystalline silicon TFTs have been actively developed for use a thin-film device for forming integrated circuits on glass substrates. A general method for forming a polycrystalline silicon film is the excimer laser method of first forming an amorphous silicon film and then irradiating the amorphous silicon film with light from an excimer laser, so as to obtain a polycrystalline silicon film by melting and re-crystallizing the amorphous silicon film.
In a commercially available laser annealing apparatus for use in the excimer laser method, a laser light beam with an irradiation aperture being around 300 mm by 0.4 mm is scanned in the short-axis direction with a pitch of several tens of xcexcm. Using this apparatus, because it is possible to form a polycrystalline silicon film in which sub-micron ordered crystal grains are randomly disposed, it is possible to mass produce thin-film transistors with a mobility of approximately 150 cm2/Vs with a high yield. In order to achieve higher performance in future TFTs, it is necessary to both increase the size of the crystal grains and control the position of the crystal grains.
In Japanese Patent No. 2689596, for example, there is disclosed a technology for achieving a polycrystalline silicon film with large coarse crystal grains, by using a two-layer amorphous silicon film to increase the grain size in the thin film part.
In this disclosure, however, there is no language whatsoever teaching the melted condition of a film based on laser irradiation conditions or a film structure other than a thickness of the film.
Additionally, there is no language that teaches with regard to crystal grain position control.
On the other hand, a progress is being made in the development of pseudo-single-crystalline silicon, by improving excimer laser annealing method and by forming crystal grain sizes that are nearly the same length as a TFT channel, while controlling the positions at which they occur.
For example, as disclosed by Im et al on page 39 of the March 1996 edition of the MRS Bulletin No. 2, by irradiating an extremely narrow line beam having a width of 5 xcexcm onto an island-shaped amorphous silicon thin-film with a pitch of 0.75 xcexcm, it is possible to form a polycrystalline silicon thin film having unidirectional growth, in which the crystal grains are substantially linearly arranged in parallelism with each other.
Additionally, as disclosed by Nakata, et al in a pre-conference publication for the 61th Society of Applied Physics Seminar (2000), No. 2, page 759, on 5p-ZD-4 and 4p-ZD-5, by using a phase-shift mask to create a laser beam having an intensity period on an order of a micron, it is possible to form position-controlled silicon crystal grains with approximately 3 xcexcm growth.
By using these methods, it will be possible to form with good control ability, a polycrystalline silicon thin film with crystal grains each having uniform large diameter at a TFT channel position.
In the case omelting these methods to control the intensity profile of a laser beam with an order of a micron, it is necessary to improve the resolution of the optics to a sub-micron order.
In doing this, however, there are the problems of an increase cost for the optics, a reduction in efficient of laser beam use, and a narrowing the depth of focus of the optics. If the depth of focus of the optics became narrower than an amount of warping or bending of the substrate, it becomes necessary to provide the substrate stage with a height-adjustment function.
It is also necessary to be able to control the movement of the substrate stage with a sub-micron order. Additionally, when using a phase-shift mask, because there is a need to have the mask be in substantially intimate contact with the amorphous silicon surface, silicon atoms that float away from the surface of the amorphous silicon film during laser annealing contaminate the mask, thereby requiring frequent replacement of a high-cost mask.
The resulting complexity of the annealing apparatus not only results in the apparatus being expensive, but also in a reduction of up-time.
Accordingly, it is an object of the present invention to provide a TFT and a method for manufacturing a TFT which simply achieves both high carrier mobility and low leakage current and the like.
In order to achieve the above-noted object, the present invention adopts the following basic technical constitution.
Specifically, a first aspect of the present invention relates to a thin-film transistor which comprising a polycrystalline silicon film layer formed on a substrate, a gate electrode formed on the polycrystalline silicon film layer via a gate insulation layer and source and drain electrodes both being arranged on both sides of the gate electrode and being connected to the polycrystalline silicon film layer, and wherein a part of the polycrystalline silicon film layer comprising a thin-film part and a thick-film part and at least a part of the thin film part being minimally used as a channel part of the transistor, and further wherein the thin-film part comprising large coarse crystal grains.
A second aspect of the present invention relates to a thin-film transistor which comprising a configuration as mentioned in the above-noted first aspect of the present invention and is further characterized in that the thick-film part comprising crystal grains a size of which is smaller than that of the large coarse crystal grains formed in the thin-film part.
A third aspect of the present invention relates to a thin-film transistor which comprising a polycrystalline silicon film layer formed on a substrate, a gate electrode formed on the polycrystalline silicon film layer via a gate insulation layer and source and drain electrodes both being arranged on both sides of the gate electrode and being connected to the polycrystalline silicon film layer, and wherein a part of the polycrystalline silicon film layer comprising a thin-film part and a thick-film part and at least a part of the thin film part being minimally used as a channel part of the transistor, and further wherein at least a portion of the thin-film part is in fully melted condition, while at least a portion of the thick-film part is in not fully melted condition.
A fourth aspect of the present invention relates to a thin-film transistor comprising:
a polycrystalline silicon film comprising a thin-film part and a thick-film part, the thin-film part being minimally used as a channel part,
wherein the polycrystalline silicon film is formed by laser annealing with an energy density that completely melts the thin-film part but does not completely melt the thick-film part.
A fifth aspect of the present invention relates to a method for manufacturing a thin-film transistor which comprising a polycrystalline silicon film layer formed on a substrate, a gate electrode formed on the polycrystalline silicon film layer via a gate insulation layer and source and drain electrodes both being arranged on both sides of the gate electrode and being connected to the polycrystalline silicon film layer, the method comprising the steps of;
forming a thin-film part and a thick-film part of an amorphous silicon film on a substrate;
polycrystallizing the thin-film part and the thick-film part by laser annealing the amorphous silicon film using an energy density completely melting the thin-film part and not completely melting the thick-film part; and
forming a thin-film transistor with the thin-film part as at least a channel part.